1. Field of the Invention
The present invention relates to a digital speed controlling apparatus, a digital motor controlling apparatus, a paper conveying apparatus, a digital speed control method, a program for making a computer execute this method, a computer-readable recording medium, and an image forming apparatus.
2. Description of the Related Art
In recent years, it is a very important task, from a viewpoint of image quality, to accurately move a paper conveying system by a desired distance and stop the system at this position, in an image forming apparatus such as a copying machine and a printer, particularly, in an image forming apparatus having an inkjet printing mechanism.
There has been developed a technique of decreasing the cost of generating a circuit and facilitating a change in control design, by digital control of software process using a low-cost general-purpose central processing unit (CPU) and a low-cost digital signal processor (DSP) in place of an analog control circuit.
A movement amount of a paper conveyor roller system that conveys recording paper in the above image forming apparatuses can be obtained at a relatively low cost, by obtaining information corresponding to a rotation angle of the conveyor roller, with a rotary encoder fitted to the conveyor roller. The encoder generates pulses with edges disposed in the encoder. While the encoder includes an analog type and a digital type, the digital type encoder can stably obtain rotation angle information with a sensor. Precision of the obtained rotation angle information depends on resolution of the encoder.
FIG. 8 is an explanatory diagram of motor control of a paper conveyor system performed by a conventional digital speed controlling apparatus. In a paper conveyor belt apparatus 7, a drive roller 372 connected to a timing belt 470 from a motor 270 rotates to move a conveyor belt 371. A rotary encoder 571 is set on the drive roller 372.
A sensor 572 detects an output from the rotary encoder 571. A digital speed controlling apparatus 17 outputs a control instruction to the motor 270, based on position information detected by the sensor 572, and applies torque to the motor 270. The control instruction has a different format, such as a current instruction and a voltage instruction, depending on the motor driver.
A control algorithm of this motor control includes a position feedback control performed near a stop position, by performing a speed feedback control up to a position near a target value. According to the position feedback control, a difference between a current position and a target position is multiplied by a predetermined gain to obtain a target speed. By feeding back the speed, a speed difference and a position difference are set to zero simultaneously.
In a conventional image forming apparatus such as an inkjet printer, a high-precision position deviation correction has not been particularly necessary as explained above. Therefore, there are not so many conventional examples of high-precision position deviation correction. Regarding other technical field, a positioning control technique of a servo control method for driving a feed rod of machine tool is proposed in Japanese Patent Application Laid-Open No. 3271440. According to this servo control method for driving a feed rod of machine tool, in order to correct a positioning error due to a backlash, a state switching time is predicted by taking a delay of a position instruction and a position output into consideration, and a servo control is performed by switching an integration time constant.
However, the technique disclosed in Japanese Patent Application Laid-Open No. 3271440 is applied for cutting metal or the like having high hardness in the machine tool and solves a similar problem in a technique assuming occurrence of a huge torque. Therefore, this technique is not suitable to control the behavior of a speed change near a stop position, as a method of correcting a positional deviation of a rotation axis of a general image forming apparatus in which a large torque like metal cutting does not occur.
FIG. 9 is a schematic diagram of one example of speed of a conveyor belt near a stop position according to the conventional digital speed controlling apparatus. After the conveyor belt is controlled to be moved to near the stop position, the conveyor belt is held near the stop position. In this case, the paper conveyor belt system is moving very slowly. As shown in FIG. 9, the conveyor belt is actually moving at a speed obtained by dividing a displacement of, for example, one-eighth pulse by a sampling cycle, at a speed equal to or smaller than a displacement that can be detected as one pulse per one sampling cycle, that is, (positional displacement of one-eighth pulse)/(sampling cycle). However, the controller cannot actually detect this move, unless the conveyer belt passes through a position at which a pulse is generated to change the value of the encoder. This is because the position is displaced by an amount smaller than a minimum unit of positional displacement.
When the pulse is deviated by one pulse from a target position by exceeding the pulse edge of the rotary encoder due to this slow move, the speed is calculated as a speed in a sampling time interval unit from a difference between a position at a current sampling time and position information at the last sampling time, as (positional displacement for one pulse)/(sampling cycle). This value is much larger than that actually obtained from (positional displacement of one-eighth pulse)/(sampling cycle). When the actual speed is slower, that is, when the control is very fine, the error becomes larger.
In other words, near the conveyor belt stop position in the image forming apparatus, when the current speed is obtained by the method of obtaining the speed in the sampling time interval unit from a difference between the position information at the current sampling time and the last position information of the sampling frequency, the obtained speed is substantially different from the actual speed. Therefore, even when the gain is switched according to the speed and the speed is fed back like in the conventional technique disclosed in Japanese Patent Application Laid-Open No. 3271440, this speed control is not valid to control the speed of a displacement equal to or smaller than a displacement (positional deviation) corresponding to one sampling frequency near the stop position.
As a result, when the speed is fed back using the speed detected in the positioning control area, the system is made unstable due to the influence of a speed error, and can even oscillate.